Characterization of transient particle loads during lithium experiments on the National Spherical Torus Experiment

Transient events such as Edge Localized Modes (ELMs) or disruptions can lead to large particle and power loads on the divertor plates of tokamak experiments. These events can cause significant erosion and are detrimental to the lifetime of the plasma facing components. Understanding the impact of ELMs remains a complex problem and a major challenge. In this study, an effort is made to characterize these ELMs and other transients based on their characteristics using a particle flux probe and surface temperature measurements from a dual-band IR camera. Typically, the temporal evolution of an ELM from the particle flux probe is characterized by a steep rise and a gradual decrease of current signal. This burst like structure is seen by the Langmuir probes as a rise in the ion saturation current with a width of a few milliseconds. This study entails gathering statistics of typical ELM-like events for various shots in order to assess the typical loading of ELMs on the Liquid Lithium Divertor (LLD) that was installed in the FY10 run campaign. Later, the power deposition profiles during ELMs are also characterized from IR camera measurements for certain discharges to find that only 15% of the energy flux arrives at the divertor target before the surface temperature reached its maximum value. Finally, a correlation was found between the particle flux from the probes during the ELMs and the neutral particle flux from Dα signal indicating the utility of the particle flux probe as a means to characterize ELMs.

► Demonstrated the use of high density Langmuir probe array for characterizing ELMs with a time resolution of 4 μs.
► A methodology was developed to characterize ELMs and ELM statistics are presented.
► An average of 1.5 MW/m2 and a maximum loading of 7.8 MW/m2 (with γ = 3) on the particle flux probe was observed during an ELM in NSTX.
► A correlation was found between particle flux from the probes with D-alpha signal during the ELMs.
► A possible convective nature of ELMs is observed in NSTX discharges with LLD operation.